A denial-of-service (DoS) or distributed denial-of-service (DDoS) attack is a malicious network activity (i.e., activity or attempt to disrupt computer operations) to make a victim (e.g., a machine or network resource) unavailable to its intended users. An attacker sends control packets to the previously-compromised flooding sources, instructing them to target at the victim. The flooding sources then collectively generate and send an excessive number of flooding packets to the victim, but with fake and randomized source addresses, so that the victim cannot locate the flooding sources.